Field of the Invention
This invention relates to safe, fire resistant, stable electrolytes for lithium primary batteries, which electrolytes include a partially or fully fluorinated, functionalized polyether with a lithium salt and a cosolvent, and may have an additional fire retardant material added. These electrolytes have performance characteristics comparable to existing highly flammable electrolytes, have a wider operating temperature span, and are non-flammable even under extreme conditions. This invention also pertains to the battery cells having said electrolytes therein.
Description of the Prior Art
It has been recognized that there is a need for safe, non-flammable primary lithium batteries, with a wide operating temperature range especially in military applications.
Prior art electrolytes that have high ionic conductivity are very flammable, due to the inclusion of low boiling point flammable solvents, which causes the batteries to explode and catch on fire if used in high temperature environments like a desert, or in enclosures without cooling or when abused and shorted. A similar problem exists with Li—SO2 catholyte batteries. Prior art electrolytes with high boiling solvents are safer, but suffer from low ionic conductivity at low temperatures. Examples are U.S. Pat. No. 5,252,413 of Alamgir et al., and U.S. Pat. No. 6,280,881 of Wendajo et al., and Patent Application No. US 2006/0204857 of Kejha et al.
To overcome these problems, the low boiling solvent electrolytes have been modified through the addition of fire-retardant additives to electrolyte formulations, such as U.S. Pat. No. 8,795,903 of Smart et al., but the additives degrade ionic conductivity and suffer compatibility issues with the electrodes. Because these additives can only be used in small quantities without significant negative impact on performance, the fire retardance conferred by their use may be inadequate for safety tests in more extreme conditions, such as puncture and bullet tests. On the other hand, fluorinated ethers (such as hydrofluoroethers, perfluorethers) and fluorinated polyethers with alkyl terminal groups have been proposed as nonflammable electrolyte solvents, due to their superior fire resistance. Examples are: US Patents Application No. US 2004/0038133 of Yamaguchi et al. To date, poor salt solubility and miscibility issues have limited their usefulness in broad applications. Similar approach has employed a dimethyl carbonate terminated long chain (10 or more polymer units) perfluoropolyether (PFPE) as the sole nonflammable electrolyte solvent in conjunction with LiTFSI as the lithium salt, as reported by DeSimone et al. from University of North Carolina—in Science Daily on Feb. 10, 2014. This improved the ionic conductivity of the fluorinated polyether, but the ionic conductivity of this system is still impractically low, and as reported, this electrolyte is directed only for use in secondary lithium ion batteries.
Instant invention overcomes these problems and provides safe, non-flammable, highly conductive electrolyte for primary batteries with a wide operating temperature range (−20° C. to 180° C.), and also provides primary battery cells with the same characteristics and many positive advantages over prior art primary batteries.